FIG. 1 is a simplified functional block diagram of conventional frequency modulated continuous-wave (FMCW) automotive radar.
The system comprises a receive antenna RAN 102, a signal conditioning unit SCU 103, a down-converter DCR 104, an analogue-to-digital converter ADC 105, a digital signal processor DSP 106, a timing/control unit TCU 111, a waveform generator WFG 110, a voltage-controlled oscillator VCO 109, acting also as an up-converter, a power amplifier/driver PAR 108 and a transmit antenna TAN 107.
The waveform generator WFG 110 supplies a control signal CV to make the voltage-controlled oscillator VCO 109 produce frequency up-sweeps and down-sweeps. Each resulting waveform SW is amplified in the power amplifier/driver PAR 108 to produce a probing waveform TW. The waveform TW transmitted by the antenna TAN 107 has a constant amplitude but its frequency sweeps the band Δf during each up-sweep or down-sweep time interval TS.
The echo RW from an obstacle OBS 101 at range R is an attenuated copy of the transmitted waveform TW, delayed in time by (2R/c), where c is the speed of light.
The echo RW is suitably processed in the signal conditioning unit SCU 103 to produce a representation AR of the reflected signal. The signal AR is combined in the down-converter DCR 104 with a copy SW of the transmitted waveform TW supplied by the voltage-controlled oscillator VCO 109. Output signal BS of the down-converter DCR 104 is first converted to a digital form DS in the analogue-to-digital converter ADC 105, and then sent to the digital signal processor DSP 106.
The digital signal processor DSP 106 receives from the timing/control unit TCU 111 a signal SS indicative of the parameters of each frequency sweep: its start time, sweep duration TS and swept frequency band Δf. The signal SS is also used by the waveform generator WFG 110 to produce a required control signal CV.
The digital signal processor DSP 106 determines the range R and velocity V of obstacle OBS 101 by analyzing beat signals BS received from the down-converter DCR 104. A beat signal BS is obtained in response to a corresponding linear frequency sweep SW of the transmitted waveform. TW; the beat frequency being defined as the frequency of a reflected wave RW minus the frequency of a transmitted wave TW.
For a stationary obstacle OBS 101, the beat-frequency magnitude |fR| is directly proportional to obstacle range R:
                                                    f            R                                    =                                            (                                                                                      Δ                    ⁢                                                                                  ⁢                    f                                                                                      T                  S                                            )                        ⁢                          (                                                2                  ·                  R                                c                            )                                =                                                    2                ·                                                                        S                    F                                                                                c                        ·            R                                              Eqn        .                                  ⁢        1            where |SF|=|Δf|/TS is the slope of a frequency sweep. The beat frequency fR is positive for frequency down-sweeps (SF<0), and negative for frequency up-sweeps (SF>0). Discrimination between positive and negative beat frequencies can be accomplished by employing quadrature signal down-conversion.
A relative movement with radial velocity V between the radar and obstacle OBS 101 will modify the ‘range-generated’ beat frequency fR by adding a Doppler frequency shift:
                              f          V                =                              2            ·            V                    λ                                    Eqn        .                                  ⁢        2            where λ is the wavelength of transmitted waveform TW. In practice, the value of Doppler shift fV will not be affected by the amount of swept frequency.
For an obstacle OBS 101 approaching the radar with velocity V, the Doppler shift fV will be positive, whereas the shift fV will be negative for an obstacle OBS 101 moving away from the radar. Consequently, the observed beat frequency fB will result from a combination of the two frequency components, fR, and fV; hence:
                              f          B                =                                            -                              (                                                      2                    ⁢                                          S                                              F                        ⁢                                                                                                                                                        c                                )                                      ·            R                    +                                    (                              2                λ                            )                        ·            V                                              Eqn        .                                  ⁢        3            
It is noted that the slope SF itself can be negative (for a down-sweep) or positive (for an up-sweep).
In the case of a single moving obstacle OBS 101, at least two frequency sweeps with substantially different slopes SF will be required to determine in a unique way both the range R and the relative velocity V of the obstacle. However, when there are two or more obstacles present in the radar's field of view (FOV), more frequency sweeps with distinct slopes will be needed to correctly determine the range and velocity of each obstacle.
From the above discussion it follows that information about ranges and velocities of obstacles is contained in the frequency components of a plurality of beat signals, each such beat signal being obtained in response to a corresponding frequency sweep transmitted by the radar.
Consequently, reliable frequency estimation carried out by the digital signal processor DSP is of primary importance.
An analysis presented in: G. M. Brooker, “Mutual Interference of Millimeter-Wave Radar Systems,” IEEE Trans. Electrarnagn. Compat., pp. 170-181, February 2007, has shown that the sweep patterns and associated signal processing techniques commonly used in automotive radar are all susceptible to multiuser interference.
Known automotive FMCW radar systems operating in multiuser environments therefore provide unreliable obstacle detection and poor estimation of ramie and velocity due to the problems caused by multiuser interference.